Why should chemicals, some of which have remarkably simple structures, have such an important effect on such a complicated and large structure as a human being? The answer lies in the way that the human body operates. If we could see inside our bodies to the molecular level, we would see a magnificent array of chemical reactions taking place, keeping the body healthy and functioning. Drugs may be mere chemicals, but they are entering a world of chemical reactions with which they interact. Therefore, there should be nothing odd in the fact that they can have an effect. The surprising thing might be that they can have such specific effects. Th is is more a result of where they act in the body—the drug targets.
As life is made up of cells, then quite clearly drugs must act on cells. The structure of a typical mammalian cell is shown in Fig. All cells in the human body contain a boundary wall called the cell membrane which encloses the contents of the cell—the cytoplasm . The cell membrane seen under the electron microscope consists of two identifiable layers, each of which is made up of an ordered row of phosphoglyceride molecules, such as phosphatidylcholine ( lecithin ). The outer layer of the membrane is made up of phosphatidylcholine, whereas the inner layer is made up of phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Each phosphoglyceride molecule consists of a small polar head-group and two long, hydrophobic (waterhating) chains. In the cell membrane, the two layers of phospholipids are arranged such that the hydrophobic tails point towards each other and form a fatty, hydrophobic centre, while the ionic head-groups are placed at the inner and outer surfaces of the cell membrane. Th is a stable structure because the ionic, hydrophilic head-groups interact with the aqueous media inside and outside the cell, whereas the hydrophobic tails maximize hydrophobic interactions with each other and are kept away from the aqueous environments. The overall result of this structure is to construct a fatty barrier between the cell’s interior and its surroundings. The membrane is not just made up of phospholipids, however.
There are a large variety of proteins situated in the cell membrane. Some proteins lie attached to the inner or the outer surface of the membrane. Others are embedded in the membrane with part of their structure exposed to one surface or both. The extent to which these proteins are embedded within the cell membrane structure depends on the types of amino acid present. Portions of protein that are embedded in the cell membrane have a large number of hydrophobic amino acids, whereas those portions that stick out from the surface have a large number of hydrophilic amino acids. Many surface proteins also have short chains of carbohydrates attached to them and are thus classed as glycoproteins. The se carbohydrate segments are important in cell Within the cytoplasm there are several structures, one of which is the nucleus. Th is acts as the ‘control centre’ for the cell. The nucleus contains the genetic code—the DNA—which acts as the blueprint for the construction of all the cell’s proteins. There are many other structures within a cell, such as the mitochondria, the Golgi apparatus, and the endoplasmic reticulum, but it is not the purpose of this book to look at the structure and function of these organelles. Suffice it to say that different drugs act on molecular targets at different locations in the cell.